Lubricating oils for sliding guide surfaces such as machine tool work tables must have low friction and anti stick-slip performance to improve machining accuracy, as well as storage stability, corrosion resistance and the like. Most machine tools have a construction in which the lubricating oil for the sliding guide surface is blended with the working fluid for the workpiece. Particularly in cases where a water-soluble cutting fluid is used as the working fluid, blending of the lubricating oil for the sliding guide surface is one cause of deterioration of the water-soluble cutting fluid (reduced cutting performance, accelerated decay, shortened mineral oil life and increased waste water disposal cost). The performance of a sliding guide surface lubricating oil must therefore include an excellent lubrication property, which reduces frictional coefficient and prevents stick-slip on the sliding guide surface, and excellent separability from the water-soluble cutting fluid, since it is blended with the water-soluble cutting fluid, without adversely affecting the performance of the water-soluble cutting fluid or of the lubricating oil for the sliding guide surface.
A variety of extreme-pressure agents or oil agents have been used to date as friction reducers. Demands for accuracy, in particular, of machine tools have been increasing in recent years, and phosphoric acid esters, acid phosphates, carboxylic acids, sulfur compounds, amines and the like have been used to realize reduced friction in the low-speed range, which has an important effect on accuracy (see Patent documents 1, 2, 3 and 4, for example). Also, neutralization of acid phosphates with alkylamines has been attempted to improve stability (see Patent document 5, for example).    [Patent document 1] Japanese Unexamined Patent Application Publication HEI No. 8-134488    [Patent document 2] Japanese Unexamined Patent Application Publication No. 2001-104973    [Patent document 3] Japanese Unexamined Patent Application Publication No. 2003-171684    [Patent document 4] Japanese Unexamined Patent Application Publication No. 2003-430949    [Patent document 5] Japanese Unexamined Patent Application Publication No. 2007-238764